Method of and system for stabilizing microwave oscillations



July 10, 1951 L. E. NORTON 2,559,730

METHOD OF AND SYSTEMS FOR STABILIZING MICROWAVE OSCILLATIONS Filed Jan. 51, 1948 3 Sheets-Sheet l CELL 21 0077 07 OUTPUT I I I I I I I P I I I I I I I I F y zawllzjwrion' July 10, 1951 L.E.NORTON METHOD OF AND SYSTEMS FOR STABILIZING MICROWAVE OSCILLATIONS Filed Jan. 51, 1948 3 Sheets-Sheet 2 11 0 c-h g? 1.5

)7 CELL sex $744 Iwerzhr Lowell .N0rl'0n July 10, 1951 E. NORTON METHOD OF AND SYSTEMS FOR STABILIZING MICROWAVE OSCILLATIONS Filed Jan. 51, 1948 '3 Sheets-Sheet 5 III I N "T N It m "a, g a

III I FI'FEQUENCV K/L OMEGWC VCLE 3 N N 6* 3 g i (O I g U 2 lnvenior' LowellENo'rion ATTORNEY Patented July 10, 1 951 UNETED STATES PATENT Q-FFEE METHOD OF AND SYSTEM STABILIZING MICRGNAVE OSCKLLATEONS poration of Delaware Application January 31, 1948, Serial No. 5,603

17 Claims. 1

This-invention relates to stabilization of the frequency of microwave oscillators and particularly relates to utilization of molecular resonance exhibited by certain gases to control the phase of the feedback between electrodes of a microwave oscillator tube.

The microwave absorption spectra of certain gases, including ammonia, carbonyl sulphide and methyl halides, comprise lines of distinctive and different frequency distribution for the different gases. At very low pressures, these lines may break up into a plurality of more sharply defined lines, each corresponding with a precise frequency not affected by changes in temperature or pressure and which, so far as has been determined, may be varied only by subjecting the gas to a strong magnetic or electric field.

In accordance with preferred forms of the invention, the feedback path of "'a microwave oscillator effectively includes a body of gas which exhibits sharp molecular resonance at the desired operating frequency of the oscillator and which upon'deviation from that frequency efiects a compensatory shift in phase of the feedback.

More particularly, and as utilized to control a -multi-acavity Klystron oscillator, the .gas may be contained ina resonant chamber external to the Klystron tube and connected in the feedback path between two of the tube cavities, so to provide an external phase shifter having an extremely high Q: in those instances where p'recise stabilization is not essential, the gas may be omitted provided the Q of the resonant chamber is sufficiently high to afford satisfactory control action. Preferably, the load circuit and the modulating circuit, if any, are coupled to electrode or cavity structure disposed in the path of the electron beam of the tube beyondthe feedback cavities'to minimize the effect of such circuits upon the stiffness of the frequency control action.

The invention further resides in methods and systems having features hereinafter described and claimed.

oscillator system using a beam deflection tube;

Figure 6 is a table showing molecular resonant frequencies of several gases; and

circuit element, due to the gas alone, is

where 'Qriequalsthe Q of the the Q of the cavity.

Figures '7 and 8 illustrate modifications of the 'Klystron oscillator circuit shown in Figure 1.

In explanation of phenomenon involved in preferred forms of the invention, it is known that there are a number of cases including NI-Is, COS,

'CI-IaOl-I, vClI-IsNI-lz and'SOz which exhibit selective line is approximately 1% when the gas pressure is of an atmosphere; is at /100 of an atmosphere, etc. However, as the pressure is further and further reduced, for example, to the order er millimeters of mercury, the absorption region breaks up into a plurality of .sharply defined lines, each precisely corresponding with a particular frequency and unaffected by any known factor except a strong magnetic or electric field. For

purposes of the present invention, this sharply resonant effect of the gas is used for stabilization of the frequency of a microwave oscillator by enclosing the .gas'in a resonant chamber included in the feedback path of the oscillator.

For the moment, assuming that such cavity has no losses at alland is filled with a nonresonant gas having an absorption coefficient (a) .of-5 10-' nepers per centimeter, the Q of this where A is equalto 1.25 centimeters.

.Such a gas, placed in a cavity with an initial Q .of'5000, gives a net Q of 2500 because gas and Q2 equals When, however, the gas absorbs microwave energy :not over a wide range of frequencies but over a narrow range which can be controlled as above stated, by varying the gas pressure so that the absorption region breaks down into a plurality of sharply defined discrete lines, the situation-is entirely different. For example, at a pres- "sure of 0.02 millimeter of mercury, the half width of the resonant curve corresponds with a Q of 40,000, and at still lower pressures, Qs as high as 100,000 may readily be obtained.

The impedance change of a resonant circuit or element in the neighborhood of its resonant frequency (in) can be represented by where Z has the dimensions of a resistance, Q is the Q of the circuit or element, and A is the incremental change of frequency.

The phase angle P) of the resonant circuit near resonance may, therefore, be expressed as When, therefore, the resonant circuit is a chamber or cavity containing gas exhibiting molecular resonance, and so has a Q upwards of 50,000, the phase angle I of the equivalent impedance (Z) varies extremely rapidly upon deviation of frequency from the resonant frequency of the gas. By way of example, a frequency deviation of only .000023 kmc. from a resonant frequency of 23.9 kmc. would for a Q of 100,000 shift the phase angle from 0 to 11 or to 349 depending upon the sense of the deviation.

The manner in which this effect may be utilized to stabilize the frequency of a Klystron oscillator will be apparent from Figure 1 and the subsequent discussion thereof. In the particular microwave oscillator shown in Figure 1, the Klystron comprises an electron gun ll, including the cathode l2 and its heater l3, which produces a beam of electrons directed toward the anode Hi. The electrode i4 is generically illustrative of the accelerating and focusing electrode structure disposed in or adjacent the path of the electron beam. In passing from the electron gun II to the anode 15, the beam traverses the resonant cavities or chambers l6, 11, each provided with a pair of spaced grids to permit passage of the beam therethrough to excite them. It is assumed that the dimensions of the cavities, the spacing between them and other circuit parameters are so chosen in accordance with known techniques, that the tube may oscillate at a frequency corresponding with a selected molecular resonance of a particular gas, for example at a frequency of 23.87 kmc. corresponding with the 3, 3 line of ammonia. Heretofore the frequency control was effected, once the cavities are adjusted to frequency, by varying the biasing voltage applied to the cathode cavity It so to control the transit time and the phase angle between the cavities, although frequency control may be effected, as shown in Figure 2, by change of the biasing potential on the grid 23 inserted between the two cavities l6 and I1. Such control was effected manually from time to time or automatically as by thermal-responsive devices but was ineffective continuously to maintain a precise frequency.

In the particular arrangement shown in Figure 1, the path between the cavities l6 and I! for feeding energy from the cavity 11 back to the cavity [6 for sustained generation of oscillations comprises a resonant chamber 18 preferably containing gas under reduced pressure and exhibiting molecular resonance at the desired operating frequency of the Klystron; the feedback path also includes a transmission line, such as pro-- vided by the concentric line sections 19 and 20, of such length that the total electrical length of the path between the cavities 16, I1 is the proper integral number of half wavelengths to provide the phase shift affording positive feedback.

The effect of a small deviation of frequency of the generated oscillations upon the phase angle of the driving current may be expressed as tan A6 where Q3 is the Q of loaded cavity H, the angle 0 is the ratio of the voltage across the loaded cavity to its driving current, and AB is an incremental change of angle 0.

Therefore the change in transit angle of the beam produced by change of any ambient or operating condition of the tube may be expressed as tan l which is readily realized when the external phase shifter utilizes the molecular resonance properties of a gas.

From the above considerations, it is apparent that when the ratio of the Q of the external phase shifter is substantially greater than the Q of the tube cavities l6, l1, and particularly that of cavity ll when coupled as by line 2| to a load circuit, the external phase shifter is effective to compensate for any condition which may tend to shift the frequency of the oscillator because the angle l of the phase shifter varies more rapidly with frequency than M of the second cavity. Any high-Q phase shifter may be used, but in the microwave region it is especially advantageous to utilize molecular resonance of a gas, as this provides a frequency standard of the highest absolute frequency precision as well as a circuit element of extremely high Q. Where such precision is not essential, the gas may be omitted, but in such event the Q of the chamber l8 itself must be substantially higher than the effective Q of cavity ii.

Preferably, cavities in addition to cavities l6 and i? are utilized for special purposes such as frequency-multiplication or frequency-modulation, or for coupling to the load circuit, to minimize effect upon the frequency control action. For example, the cavity 22, Figure 3, provided for any of these purposes, is distinct from the cavities l6 and 17B utilized for generation of oscillations of predetermined frequency by Klystron 503. Such additional cavity or cavities should be disposed in the path of the electron beam beyond the feedback cavities l6 and 11B, that is, the additional cavity or cavities should be more remote from the electron gun H than the feedback cavities and in any event should not be between cavities IB, NB.

The same rigid frequency-control action may be obtained by inserting the external phase shifter 18 anywhere between two feedback cavities of a Klystron, for example, the gas cell !8, Figure 4, may be inserted in a transmission line coupling the electron beam to the external cavity HA of Klystron HJA. Operation is the same as that described for the arrangement of Figure 1 provided the high Q phase shifting element is placed anywhere in the loop connecting the two cavities Hi and I1.

The invention may also be applied to microwave oscillator tubes other than those of the Klystron type; for example, as shown in Figure 5 it may be utilized to control the frequency of a microwave oscillator using a beam-deflection tube lllD whose construction may be the same or generally similar to that shown in copending application Serial No. 759,769 filed July 9, 1947, now Patent Number 2,551,810, by Charles W. Mueller and entiled Deflection Beam Tube. Briefly, the electron beam issuing from the cathode gun H passes between a pair of electrodes |6d subjected to an alternating difference of potential from the feedback loop. The loop includes the high Q phase-shifter l8. In crystal multiplier I, n is chosen so that 71.) falls on a gas resonance line. The output of crystal multiplier 2 is (nil) f. The useful output of the crystal mixer is again ,1. This potential at frequency f is applied to the input deflection plates and this input frequency is, of course, the same as the output frequency of the deflection tube. Preferably, the external phase-shifter I 8, as in preceding figures, is a resonant gas-tight chamber containing, under pressure of less than a millimeter of mercury, a gas exhibiting molecular resonance at the desired harmonic n of the operating frequency f of the oscillator. As in other oscillators of this type, the electron beam as periodically deflected by electrodes [6D sweeps across the thin wire electrode 25 to provide a series of timed impulses which through the feedback path provides for continued generation of oscillations. The defiection plate is preferably biased by a unidirectional voltage so that the beam impinges upon electrode 25 only once per cycle.

With respect to Figure 5, this and other arrangements for stabilizing at a harmonic of the oscillator frequency are described and claimed in application Serial No. 218,808, filed April 2, 1951, entitled Stabilization of Oscillators from High Frequency Standards.

In each of the modifications shown in Figures '7 and 8, there is utilized a feedback loop which is matched, or approximately so, except for a discontinuity in its pass characteristics due to molecular resonance of gas in the gas cell IGA, Figure 7, or gas cell IBB, Figure 8.

The modifications shown in Figure '7 and Figure 8 indicate alternative manners in which the gas cell may be coupled in a branch arm of the feedback transmission loop.

The species of the invention illustrated in Figure 5 of the drawing is disclosed and claimed in a copending continuation-in-part application Serial No. 140,813, filed January 27, 1950, now abandoned entitled Methods of and Systems for Stabilizing Microwave Oscillation and assigned to the same assignor as the instant application.

What is claimed is:

1. The method of utilizing the molecular resonance characteristics of a microwave absorptive gas to stabilize the frequency of a microwave generator having a feedback loop which comprises impressing the feedback energy upon a body of gas exhibiting molecular resonance at the desired operating frequency of said generator, and controlling the phase of the feedback by the resulting impedance efiects of said gas.

2. The method of utilizing the molecular resonance characteristics of a microwave absorptive gas to stabilize the frequency of a microwave generator having a feedback loop whose effective electrical length determines the phaseshift of the feedback energy which comprises passing at least part of said energy through a body of gas exhibiting molecular resonance at the desired operating frequency of said generator to vary the effective length of said feedback loop in compensation for deviations from said desired operating frequency.

3. The method of utilizing the molecular resonance characteristic of a microwave absorptive gas to produce microwave oscillations of stabilized frequency by a multi-cavity Klystron which comprises coupling two cavities of said Klystron to provide a feedback path for generation of self-sustained oscillations, and coupling said feedback path to a body of gas exhibiting molecular resonance at the desired frequency of said oscillations to vary the phase shift of the energy in said path in compensation for deviations from said frequency.

4. The method of utilizing the molecular resonance characteristics of a microwave absorptive gas to stabilize the frequency of a microwave generator having a feedback loop which comprises adjusting the pressure of a body of gas exhibiting sharp molecular resonance at, the desired operating frequency of said generator to the order of millimeters, and impressing the feedback energy upon said gas to produce marked shift in phase of said energy upon deviations from said desired operating frequency.

5. The method of utilizing the molecular resonance characteristics of a microwave absorptive gas to produce microwave oscillations of stabilized frequency by a multi-cavity Klystron which comprises adjusting the pressure of a body of gas exhibiting sharp molecular resonance at the desired frequency of said oscillations to the order of millimeters, coupling two cavities of said Klystron to provide a feedback path for generation of self-sustained oscillations, and coupling said path to said body of gas to effect thereby control of the phase of the feedback energy in compensation for deviations from said desired frequency.

6. The method of employing the molecular resonance characteristics of a body of microwave absorptive gas for effecting stabilization of the frequency of a multi-cavity Klystron oscillator which comprises externally connecting between two of the cavities a phase-shifter including said body of said gas, the ratio of whose Q to the Q of the loaded cavity of the Klystron is very much greater than unity, and adjusting the electrical length of the path including said phase shifter to such magnitude that an incremental change in frequency of the oscillator produces a compensatory change in phase of the feedback through said path.

7. The method of employing the molecular resonance characteristics of a body of microwave absorptive gas for operating a multi-cavity Klystron to produce oscillations of stable frequency which comprises coupling the load to a cavity remote from the beam-forming electrodes, externally connecting between two other cavities successively traversed by the beam in passing from the cathode to the load cavity a phaseshifter including said body of gas whose Q is substantially higher than the Q of either of said two other cavities, and adjusting the electrical length of the path including said phase-shifter to such magnitude that an incremental change in frequency of the oscillator produces a compensatory shift in phase of the feedback through said path.

8. A frequency-stabilized microwave oscillator system comprising an oscillator tube having at least two resonant cavities, a transmission line coupling two of said cavities for feedback of energy sustaining generation of oscillations, and a cell containing gas exhibiting molecular resonance at the desired frequency of said oscillations coupled to said line and upon which the feedback energy is impressed to control the phaseshift thereof by reactive effects of the gas.

9. A frequency-stabilized oscillator system comprising an oscillator tube having at least two resonant cavities, a transmission line coupling two of said cavities for feedback of energy sustaining' generator of oscillations, and a cell included in said transmission line and including gas exhibiting molecular resonance at the desired frequency of said oscillations.

f lO. A frequency-stabilized oscillator system comprising an oscillator tube having at least two resonant cavities, a transmission line coupling two of said cavities for feedback of energy sustaining generation of oscillations, and a cell coupled to said transmission line and including gas exhibiting molecular resonance at the desired frequency of said oscillations.

11. A system according to claim 3 wherein said cell is serially connected in said line.

12. A system according to claim 11 includingv a control electrode operatively disposed in said tube between said resonant cavities, and means for adjusting the potential of said electrode with respect to said cavities.

13. A system according to claim 8 including a load resonant cavity remote from the beamforming elements in said tube, and wherein said two cavities coupled to said line are operatively disposed between said beam-forming elements and said load cavitiy.

14'. A system according to claim 8 wherein one of said cavities is serially connected with said gas cell in said line externally of said tube,

15. A system according to claim 8 wherein said cell is coupled to said line through an impedance transforming device.

16. A system according to claim 8 wherein said cell is coupled in shunt with a portion of said line.

17. The method of utilizing the molecular resonance characteristics of a microwave absorptive gas to stabilize the frequency of a microwave generator having a feedback loop which comprises impressing the feedback encrgy upon a body of gas exhibiting molecular resonance at an integral multiple including unity of the desired operating frequency of said generator, and controlling the phase of the feedback by the resulting impedance effects of said gas.

LOWELL E. NORTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,281,935 Hansen et al May 5, 1942 2,445,811 Varian July 2'7, 1948 OTHER REFERENCES RCA pamphlet entitled The Absorption of Micro-Waves by Gases, reprinted from Journal of Applied Physics, vol. 17, No. 6, June 1946.

RCA pamphlet entitled Absorption of Microwaves by Gases, II, reprinted from Journal of Applied Physics, vol. 1'7, No. 10, October 1946. 

